5-trans PGF2α is the more thermodynamically stable C-5 olefin isomer of PGF2α and is a common impurity in commercial lots of PGF2α. 5-trans PGF2α administered intravenously to anesthetized rabbits caused a substantial (ten-fold) increase in respiratory rate, but this attribute was common to a number of F-series compounds and analogs.{2224}